#include <bits/stdc++.h>

using namespace std;

vector<vector<int>> Go(int n) {
  int K = __countr_zero(__bit_ceil(n));
  vector ret(K + 1, vector<int>(n, n + 1));
  ret[0][0] = n;
  for (int k = 1; k <= K; ++k) {
    int r = 0;
    for (int l = 0; l < n; ++l) {
      r = max(r, l + 1);
      while (r < ret[k - 1][max(2 * l - r, 0)] && min(2 * r - l, n) < ret[k - 1][l]) {
        ++r;
      }
      ret[k][l] = r;
    }
  }
  return ret;
}

void Solve() {
  int n, m;
  cin >> n >> m;
  vector<string> s(n);
  for (int i = 0; i < n; ++i) {
    cin >> s[i];
  }

  vector sum(m, vector<int>(n + 1));
  for (int j = 0; j < m; ++j) {
    for (int i = n; i--;) {
      sum[j][i] = (s[i][j] == '1') + sum[j][i + 1];
    }
  }

  auto vn = Go(n);
  auto vm = Go(m);

  int ans = 1e9;
  for (int kn = 0; kn < int(vn.size()); ++kn) {
    for (int l = 0; l < n; ++l) {
      if (n < vn[kn][l]) {
        break;
      }
      int r = vn[kn][l];
      int nj = m;
      int km = int(vm.size());
      for (int j = m; j--;) {
        if (sum[j][l] - sum[j][r] == r - l) {
          while (km && vm[km - 1][j] <= nj) {
            --km;
          }
          if (km < int(vm.size())) {
            ans = min(ans, kn + km);
          }
        } else {
          nj = j;
        }
      }
    }
  }
  cout << ans << '\n';
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  Solve();
}